1. Field of the Invention
This invention relates generally to supports for wafers in semiconductor processing chambers and, more particularly, to a wafer holder for supporting a wafer within a cold wall chemical vapor deposition chamber.
2. Description of the Related Art
High-temperature ovens, or reactors, are used to process semiconductor wafers from which integrated circuits are made for the electronics industry. A substrate, typically a circular silicon wafer, is placed on a wafer holder. If the wafer holder helps to attract heat, it is called a susceptor. The wafer and wafer holder are enclosed in a quartz chamber and heated to high temperatures, such as 600xc2x0 C. (1112xc2x0 F.) or higher, by a plurality of radiant lamps placed around the quartz chamber. A reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes in other equipment, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits.
If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure.
Various CVD process parameters must be carefully controlled to ensure the high quality of the deposited films and the resulting semiconductor. One such critical parameter is the temperature of the wafer during the processing. The deposition gas reacts at particular temperatures and deposits on the wafer. If the temperature varies greatly across the surface of the wafer, uneven deposition of the reactant gas occurs. Similarly, temperature uniformity can be important for a variety of other semiconductor fabrication processes, such as etching, annealing, doping, etc.
Rotatable wafer holders are known in the art. Rotation of the wafer holder results in more uniform temperature distribution and deposition across the wafer.
In recent years, single-wafer processing of larger diameter wafers has grown for a variety of reasons including its greater precision as opposed to processing batches of wafers at the same time. Although single-wafer processing by itself provides advantages over batch processing, control of process parameters and throughput remains critical. In systems in which the wafer is supported in intimate contact with a large-mass, slab-like susceptor, the necessity of maintaining uniform susceptor temperature during heat-up and cool-down cycles limits the rate at which the temperature could be changed. For example, in order to maintain temperature uniformity across the susceptor, the power input to the edges of the susceptor had to be significantly greater than the power input to the center due to the edge effects.
As explained above, CVD processing often occurs at temperatures of 600xc2x0 C. (1112xc2x0 F.) or higher. One common problem associated with CVD processing is that, when a cold wafer is loaded onto the top surface of a susceptor inside a pre-heated reaction chamber, the wafer tends to experience xe2x80x9cthermal shockxe2x80x9d due to thermal gradients within the wafer from sudden conductive heat transfer from the hot susceptor to the cold wafer. These thermal stresses can result in wafer xe2x80x9ccurlxe2x80x9d and xe2x80x9cpop,xe2x80x9d as well as damage to the backside of the wafer. The largest problem associated with such thermal gradients is wafer pop, which causes the wafer to move randomly on the susceptor surface. This movement causes temperature non-uniformities, which reduces the repeatability of process characteristics such as thickness uniformity.
One method to reduce the problems associated with thermal shock is to substantially decrease wafer load temperatures. This is not common because it adversely affects throughput, since the temperature must be decreased before each new wafer is loaded and then increased before processing of the wafer can begin. Decreases in throughput results in decreased production and greater manufacturing costs. Thus, in order to maintain a desired throughput, some degree of wafer curl and pop are usually tolerated.
Some susceptors are equipped with vertically oriented lift pins that are vertically moveable through holes in the surface of the susceptor upon which the wafer rests. When the lift pins are elevated, the wafer is separated from the susceptor surface so as to slow heat transfer from the susceptor to the wafer. This permits the wafer to preheat, thus reducing thermal gradients when the wafer is lowered into contact with the susceptor. When the lift pins are lowered, the wafer is brought into flush contact with or very close to the susceptor surface, permitting conductive heat transfer therebetween.
Presently, there is a need for an improved wafer support system, which permits higher wafer load temperatures while avoiding the problems associated with thermal shock.
It is a principle object and advantage of the present invention to provide an improved wafer support system that permits a cold wafer to be loaded into a pre-heated reaction chamber while avoiding the above-mentioned problems associated with thermal shock.
Prior art susceptors utilizing lift pins can reduce the effects of thermal shock by permitting the loading of a wafer onto the lift pins in their elevated positions. When the wafer is in the elevated position, the wafer temperature can be permitted to increase much more gradually than would occur if the wafer were immediately brought into flush contact with a flat surface of the susceptor. In order to reduce the risk of thermal shock, the wafer can be maintained in the elevated position until the wafer temperature increases to a degree such that the likelihood of thermal shock is substantially reduced or eliminated when the wafer is eventually lowered onto the susceptor surface.
One problem with lift pins is that they tend to scratch the lower surface of the wafer, which in turn increases the likelihood of crystallographic slip. Slip is a defect in the crystalline structure of the wafer, which destroys any devices through which it may pass. The presence of scratches on a wafer causes slip to occur in the wafer at lower temperatures than if no scratches are present. In other words, the presence of scratches makes a wafer less robust and less able to tolerate high temperatures. Scratches also increase the susceptibility of a wafer to slip under rapidly varying temperature conditions. In addition, scratches cause nodule growth on the backside of the wafer, which leads to alignment problems during photolithography.
The preferred embodiments of the present invention solve this problem by providing a vertically moveable lift ring that supports the outer radial periphery of a wafer. The lift ring can be raised above the remaining portions of the susceptor to receive a newly loaded wafer. In the raised position, the only heat conduction received by the wafer is via the peripheral lift ring. Heat conduction between the remainder of the susceptor and the raised wafer is substantially prevented. Moreover, heat conduction to the wafer is localized at the wafer periphery, preferably in the exclusion zone of the wafer. When the wafer is in the raised position, the majority of the heat received by the wafer is in the form of (1) radiation from the heated susceptor (from both the lift ring and the remainder of the susceptor), (2) radiation from the heat lamps, if they are on, and (3) convection from warm gas within the chamber. In addition, as indicated above, some heat is received at the wafer edge in the form of conduction from the lift ring. The wafer can be maintained in the raised position until the wafer temperature rises to a level sufficient to substantially prevent or significantly reduce thermal shock to the wafer when the wafer is eventually lowered onto the remainder of the susceptor. The lift ring provides more stable support to the wafer than lift pins. Further, in contrast to lift pins, the lift ring does not have any sharp contact surfaces that might scratch the wafer. The upper surface of the lift ring is preferably flat and only contacts the wafer near its outer radial edge.
As used herein, heat xe2x80x9cconductionxe2x80x9d refers to the transfer of energy arising from the temperature difference between adjacent bodies. It is not uncommon for skilled artisans to understand heat conduction to include heat transfer across small gaps. However, for the purposes of the present application, xe2x80x9cconductionxe2x80x9d does not include heat transfer across small gaps.
In one aspect, the present invention provides a susceptor for supporting a wafer within a reaction chamber, comprising an inner plug, a lift ring, and a lift device. The inner plug can have a generally flat top surface, a gridded surface including grooves, a concave gridded surface, or other configuration. The lift ring has an upper wafer support surface configured to support a bottom outer peripheral surface of a wafer. The lift ring also has a central aperture positioned such that the lift ring contacts only a peripheral portion of a wafer supported thereon. The central aperture is sized and shaped to closely receive the inner plug. The lift ring has a lowered position in which the wafer support surface is generally at the same vertical position as a top surface of the inner plug. The lift ring also has an elevated position in which the wafer support surface is above the top surface of the inner plug such that a wafer supported on the lift ring substantially does not contact the inner plug. The lift device operates to move the lift ring between its lowered position and its elevated position.
In another aspect, the present invention provides a reactor having a susceptor as described in the previous paragraph.
In another aspect, the present invention provides a support spider for supporting a susceptor such as the one described above. The support spider comprises a generally vertical shaft having a vertical center axis, a plurality of support arms extending from the shaft, and intermediate support members. The support arms comprise generally horizontal portions and generally vertical portions. The horizontal portions extend generally radially outward from the shaft to outer ends, and the vertical portions extend generally upward from the outer ends of the horizontal portions. The vertical portions have upper ends configured to be underneath the lift ring when the vertical center axis of the shaft is generally aligned with a vertical center axis of the susceptor. The intermediate support members extend generally vertically from the horizontal portions, are positioned radially inward of the vertical portions, and have upper ends positioned below the upper ends of the vertical portions. The spider is configured to be positioned underneath the susceptor such that the spider can be rotatated about the center axis of the shaft and vertically displaced. When the center axes of the shaft and the susceptor are generally aligned, the spider has a position in which an upward displacement of the spider of a first distance causes the vertical portions to lift the lift ring above the inner plug without the inner plug being lifted. An upward displacement of the spider beyond the first distance causes the intermediate support members to lift the inner plug while the vertical portions support the lift ring above the inner plug.
In another aspect, the present invention provides a method of processing a wafer on a susceptor such as the one described above, within a processing chamber. In this method, the lift device comprises a support spider having arms extending radially outward and upward to contact a bottom surface of the susceptor. The spider is vertically moveable and rotatable about a vertical axis. According to the method, with the lift ring in the lowered position thereof, the inner plug is supported on the spider. The spider is lowered to a position such that the inner plug becomes supported on transition support members of the processing chamber, the lift ring still supported on the inner plug. The spider is rotated such that its arms are underneath and positioned to contact portions of the lift ring but not the inner plug if the spider is elevated. The spider is elevated such that the lift ring moves to the elevated position thereof. A wafer is loaded onto the lift ring in the elevated position thereof. The temperature of the wafer is permitted to increase to a level sufficient to substantially minimize thermal shock to the wafer when the wafer is placed into contact with the top surface of the inner plug. The spider is lowered such that the lift ring moves to the lowered position thereof and the wafer becomes supported on the top surface of the inner plug.
In another aspect, the present invention provides an apparatus for supporting a wafer within a reaction chamber. The apparatus comprises an inner portion, a lift ring, and a lift device. The inner portion has a top wafer support surface configured to support a bottom central surface of a wafer. The lift ring has an upper wafer support surface configured to support a bottom outer peripheral surface of a wafer. The lift device operates to move the lift ring vertically with respect to the inner portion.
In yet another aspect, the present invention provides a method of loading a wafer into a processing chamber having a temperature higher than that of the wafer. According to the method, a wafer is positioned onto a lift ring within a processing chamber such that a bottom outer peripheral surface of the wafer is supported by an upper wafer support surface of the lift ring. The lift ring has a central aperture configured so that substantially only a peripheral portion of the wafer is in contact with the lift ring. The lift ring is lowered into surrounding engagement with an inner plug having a top surface, so that the inner plug is positioned within the central aperture of the lift ring. In the lowered position of the lift ring, the top surface of the inner plug and the upper wafer support surface of the lift ring are generally coplanar, at least one of such surfaces supporting a bottom surface of the wafer.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above and as further described below. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.